as the others have said, it basically charges you for time left in the billing cycle. i just switched to verizon from at&t. they began my service immediately, but there were only two days left in my new verizon fixed cycle. so on my first bill, they charged me for 2 days + my whole next month. in those two days, i had my full bucket of minutes, messages and data, but was only charged $5.95 ($2.95 per day).
lets say your plan is $100 a month and your cycle, for easy sake, is 10/1-10/31. lets say you sign up for service on 10/15. they cant charge you $100 (a whole months worth) if you will only be using a half month. therefore, they prorate it and only charge you $50. now some carriers may decide to also prorate your minutes and messages, depending on their terms. at&t did this to me years ago. so lets say your plan is for 900 minutes/month. prorated, they will only be offering you 450 of those for the rest of your cycle. but now with the new share plans, they are unlimited so it isnt an issue. got it?